Implementations use names starting with an underscore followed by an uppercase letter or two underscores to avoid conflicts with user-defined macros. Such names are reserved in C++.
For example, one could define a macro called Type and then #include <vector>. If vector implementations used Type as a template parameter name, it would break.
However, one is not allowed to define macros called _Type (or __type, type__ etc.). Therefore, vector can safely use such names.